MTB: downtube cable routing

I've seen it done on some full-suspension frames, and it seems to work
well at least in dryish conditions. I'm not sure about serious amounts
of mud though. Does the frame use housing to run the cables under the
BB, or a road bike style cable guide?

There appears to be a grooved cable guide, so we're talking exposed
cable. Given the amount of crud that forms on my current MTB in that
area, I'm a bit hesitant.

As for rocks, I wouldn't be too worried. Your odds of bashing the
underside of the BB shell on a rock without destroying your chainrings
in the process are not terribly high.

Agreed. That was just an example (and a bad one). Loose/flying rocks
are much more of a problem (my current downtube is full of nicks,
presumably from rocks). I live in a wet climate, so even on-road
riding results in more-than-expected grime.